Martine Talbaut

Comparison of Numerical Studies Characterizing Optical Properties of Soot Aggregates for Improved EXSCA Measurements

In order to compare EXSCA measurements with light-scattering calculations, numerical studies characterizing the optical properties of soot aggregates were compared by using different approaches: [1] the Rayleigh-Debye-Gans theory for the fractal aggregate model (RDG-FA), studied by Faeth and Koylu, [2] the rigorous solution model (RS) proposed by Xu, and [3] the discrete dipole approximation model (DDA), developed by Draine and Flatau. The extinction, absorption and scattering cross-sections, C, C and C, and matrix scattering coefficients, |S1|2, |S2|2, |S3|2 and |S4|2, were studied, emphasizing the extinction coefficient C and the scattering coefficient |S1(90°)|2. First, these coefficients for a panel of six aggregates with 64 or 128 primary spheres were compared using the three models. For the absorption and extinction cross-sections, the results are close and RDG-FA may be adequate to determine these parameters. For the total scattering cross-section, the DDA model is close to RS whereas the RDG-FA model shows limitations with strong relative differences. For the scattering coefficients, we focused on |S1|2 and |S2|2, |S3|2 and |S4|2 being negligible. For the 64-sphere aggregates, the relative differences between DDA and RDG-FA are generally great and higher for RDA-FA than for DDA. These deviations are especially significant for backscattering. If, on the contrary, we focus on |S1(90°)|2, all of the models give a good prediction. To complete this study, computation times for DDA and RS are indicated and cross-section distributions for a panel of 28 aggregates obtained using RDG-FA and DDA are presented.

Inversion method and experiment to determine the soot refractive index: application to turbulent diffusion flames

Experimental and numerical studies have been performed to determine the soot refractive index in methane turbulent diffusion flames with two oxidizers: air and oxygen. In the flame zone, soot particles were sampled with a cooled probe. Measurements of optical soot properties have been carried out to obtain extinction and vertical-vertical (90°) scattering coefficients. The size distributions were obtained by electrical mobility analysis. Using these distributions, the optical properties have been computed with the Rayleigh-Debye-Gans theory for fractal aggregates by considering the morphology of soot aggregates and using morphological parameter values based on literature reports for other similar systems. Then, the refractive index has been obtained from a numerical inversion method by matching the measured and computed optical coefficients. This refractive index determination method is new to our knowledge. In turbulent diffusion methane oxygen flames the soot refractive index averaged value found is m = 1.95(±0.13)-0.51i(±0.12), and in the air flame m = 2.10(±0.12)-0.48i(±0.06). In view of the uncertainties, the refractive index is independent of the oxidizer type, the aerodynamic conditions and the flame zone location for the sampling. A sensitivity analysis has been carried out to study the influence of some morphological and experimental parameters on the refractive index value.

Impact of CO2/N2/Ar Addition on the Internal Structure and Stability of Nonpremixed CH4/Air Flames at Lifting

The authors focused on how adding CO2 to the air influences the transition from an attached flame to a lifted flame issued from a coaxial nonpremixed methane-air jet. To discriminate between effects due to a diluent (dilution, thermal, or chemical impacts), chemically and thermally inert N2 and chemically inert Ar were also investigated. Flame lifting always occurs, essentially controlled by the critical flow-rate ratio, (Qdiluent/Qair)lifting. CO2 has the strongest ability to break flame stability, followed by N2, then by Ar. A unique attachment height and OH thickness characterize lifting for all the diluents; lifting is attained once the same critical flame edge propagation speed is reached. (Qdiluent/Qair)/(Qdiluent/Qair)lifting is the affine parameter of similarity laws describing Ha and EpOH evolutions with dilution. Aerodynamics competes with dilution to impose lifting and boundary effects cannot be ignored in a fine analysis. The flame behaves differently according to whether lifting results from aerodynamics or dilution.